This application claims priority to prior application JP 2001-387060, the disclosure of which is incorporated herein by reference.
This invention relates to a digital radio receiver for receiving a radio wave from an artificial satellite (which may be called a xe2x80x9csatellite wavexe2x80x9d hereinafter) or a radio wave from a ground station (which may be called a xe2x80x9cground wavexe2x80x9d hereinafter) to listen to digital radio broadcasting and, in particular, to a composite antenna apparatus for use in the digital radio receiver.
In recent years, a digital radio receiver for receiving a radio wave from an artificial satellite (satellite wave) or a ground wave to listen to digital radio broadcasting has been developed and is about to be put into practical use in United States of America. The digital radio receiver is mounted on a mobile station, such as a vehicle, and is adapted to receive a radio wave having a frequency of about 2.3 GHz to listen to the digital radio broadcasting. In other words, the digital radio receiver is a radio receiver capable of listening to mobile broadcasting. It is noted here that the ground wave is a radio wave obtained by slightly shifting the frequency of the satellite wave after it is received by the ground station.
In order to receive the radio wave having the frequency of about 2.3 GHz, it is necessary to mount an antenna at a position outside a vehicle. Such antenna may have various structures but generally has a stick-type structure rather than a planar-type (or a flat-type) structure.
As well known, an electromagnetic wave emitted into a free space is a transversal wave having an electric field and a magnetic field vibrating or oscillating in a plane perpendicular to a propagating direction of the wave. In some circumstances, the oscillation of the electric field and the magnetic field is restricted to a specific direction. Such nature is referred to as polarization and such wave is called a polarized wave. The satellite wave uses a circular polarized wave exhibiting circular polarization while the ground wave uses a linear polarized wave exhibiting linear polarization.
Hereinafter, description will mainly be made about an antenna for receiving the satellite wave. As one of stick-type antennas, a helical antenna is known. The helical antenna comprises a hollow or solid cylindrical member and at least one conductor wire wound around the cylindrical member in a helix pattern (or a spiral pattern). The helical antenna can efficiently receive the above-mentioned circular polarized wave. Therefore, the helical antenna is frequently used to receive the satellite wave. The cylindrical member is made of an insulating material such as plastic. The number of conductor wires is equal to, for example, four. Practically, it is very difficult to wind at least one conductor wire around the cylindrical member in a helix pattern. Instead, proposal is made of a structure in which an insulating film with at least one conductor pattern printed thereon is wound around the cylindrical member.
Referring to FIGS. 1 and 2, an existing composite antenna apparatus comprises a monopole antenna 11 having a finite ground plane and disposed on a circuit board 12, and a cylindrical member 14 with a plurality of conductor patterns 13 formed on its peripheral surface and extending in a helix pattern. A combination of the cylindrical member 14 and the conductor patterns 13 forms a helical antenna. The cylindrical member 14 is formed by an insulating film rolled into a cylindrical shape and fixed to keep the cylindrical shape. In the composite antenna apparatus, the finite ground plane has a radius equal to xc2xc wavelength and the monopole antenna 11 has a length equal to xc2xc wavelength. With the above-mentioned structure, the capacitance is large under the influence of the helical antenna around the monopole antenna 11 so that impedance matching is difficult. Therefore, in the existing composite antenna apparatus having the above-mentioned structure, it is necessary to provide a matching circuit 15 connected through a lead wire 16 to the circuit board 12, as shown in FIG. 3. The matching circuit 15 is disposed outside the composite antenna apparatus comprising the monopole antenna 11, the circuit board 12, and the cylindrical member 14. Therefore, the presence of the matching circuit is a bottleneck against miniaturization of the composite antenna apparatus.
It is an object of this invention to provide an antenna apparatus which itself has a function of a matching circuit so that the antenna apparatus is reduced in size.
According to this invention, there is provided a composite antenna apparatus comprising a cylindrical member formed by a flexible insulating member rolled into a cylindrical shape, a circuit board fixed to one axial end of the cylindrical member and provided with a first metallic pattern, and a monopole antenna disposed inside the cylindrical member and standing up on one surface of the circuit board, wherein the circuit board has a second metallic pattern formed on the other surface thereof, the monopole antenna having a coil portion wound in a spiral fashion.
In the above-mentioned composite antenna apparatus, the coil portion serves as an inductor. The monopole antenna serves as a resistor by its length. The first and the second metallic patterns serve as a capacitor. A combination of the coil portion, the monopole antenna, and the first and the second metallic patterns forms an RLC circuit which serves as a matching circuit.